This application claims the benefit of the priority date of German application DE 100 44 257.9, filed on Sep. 7, 2000, the contents of which are herein incorporated by reference.
The invention relates to lithographic processing, and in particular, to simulation of lithographic processes.
In the practice of lithography, original data defining an original layout are prescribed. New data are then automatically calculated proceeding from the original data. The calculation is effected in such a way that the new data define a new mask layout whose geometry is more similar to a mask produced or producible using the original data than it is to the original layout.
In the case of a known method carried out with the aid of the xe2x80x9cSelidxe2x80x9d program from Sigma-C, the successive steps of the production process for producing a photomask are simulated. Such steps include: writing the mask to a photoresist by means of a laser or electron beam writer; developing the photoresist; etching the mask; and performing reaction diffusion processes. The simulation of the mask production process requires an additional program that differs from the program used later for simulating exposure and resist development processes in a wafer. This additional program requires additional input parameters, some of which have to be determined experimentally in a complicated manner. Simulating the steps of the production process therefore requires additional expenditure of time and computation complexity, and significant data processing capability.
It is an object of the invention to specify a simple method for generating mask layout data for lithography masks, in which method the new layout, with a reduced outlay, continues to be very similar to a mask that is produced using the original data. Moreover, the intention is to specify an associated apparatus and an associated program.
The invention is based on the recognition that deviations between a mask defined by a layout and either a mask produced according to this layout or a mask modeled proceeding from this layout with simulation of the production process can be attributed to the production process. These deviations depend on the geometry of the mask to be produced and can largely already be predicted on the basis of the geometry of the original layout. This makes it possible to rapidly take account of the influences of the production process while avoiding the need to simulate the individual steps of the production process
In the case of the method according to the invention, in addition to the method steps mentioned in the introduction, the new data which are intended to be used for the lithography simulation are calculated on the basis of rules which are based on deviations in the geometry of a layout from a mask that is produced according to this layout. As an alternative to the production, the mask that is used for comparison purposes can also be modeled proceeding from the layout with simulation of the steps of the production process. In both cases, in the method according to the invention, the individual method steps of the production process of the mask are not simulated, however, during the calculation of the new data. The inputting of a multiplicity of process parameters for the simulation of the production process and the computationally complicated simulation itself are thus obviated.
The deviations in the geometry can be calculated by means of simple geometric relationships. Differential equations, such as e.g. diffusion equations, do not have to be solved. As a result, the new mask data can be calculated with a computation complexity that is reduced by orders of magnitude in comparison with the simulation of the production process.
In a development of the method according to the invention, the rules are geometric calculation specifications for defining the boundaries of a structure at a position of the new layout depending on the length and/or the area of a reference structure located at the same position in the original layout. Alternatively or cumulatively, the distance between the reference structure and the adjacent structures of the original layout is also included in the calculation specification. The length and the area of a structure determine the extent of the deviations to be determined. The adjacent structures allow conclusions to be drawn regarding the locations at which deviations will occur. This is because different effects occur during the mask writing process of closely adjacent structures than during the writing of adjacent structures at a greater distance from the reference structure.
In one refinement, in accordance with a rule, a shortening value is determined for an elongate reference structure, which is also referred to as a line structure or as a line for short, at a position of the original layout. Depending on the shortening value, the structure located at the same position in the new layout is shortened in the longitudinal direction in comparison with the reference structure. The line shortening can be attributed to the absence of adjacent structures. Thus, the nature of the line shortening and the extent of the line shortening can be determined on the basis of the geometry of the original layout. Instances of line shortening are illustrated in FIGS. 2 to 4, which are explained in more detail below.
In another refinement, in accordance with a further rule, a cornered reference structure is determined at a position of the original layout. A corner can be defined with the aid of the angle between two meeting lines or straight edges. In a customary design the structures often have corners whose edges are at an angle of 90xc2x0 with respect to one another (however, any desired angles are also conceivable). For the purpose of rounding a corner, at least one radius or curvature value is determined or input by an operator. Depending on the radius or curvature value, the new data are calculated in such a way that the structure located at the same position in the new layout has a rounded edge profile instead of the corner. The radius value can be determined, for example, directly from the width of a structure. The circle equation, for example, can then be used for calculating the position of the structure in the new layout. Instances of corner rounding are explained below with reference to FIGS. 2 to 5.
In one refinement, the radius is chosen depending on the surroundings. In the case of a light-absorbing structure which is arranged around a light-transmissive structure, an inner corner is rounded with a smaller radius than an outer corner of the light-absorbing structure.
In a further refinement, in accordance with a rule, a constriction value is determined for an elongate reference structure at a position of the original layout. Depending on the constriction value, the structure located at the same position in the new layout is then constricted at least in sections transversely with respect to the longitudinal direction in comparison with the reference structure. This measure takes account of the so-called xe2x80x9cpeanutsxe2x80x9d effect because constrictions of structures are simulated which can be attributed to the absence of adjacent structures during the production of the mask. The xe2x80x9cpeanutsxe2x80x9d effect is explained below with reference to FIG. 7.
In a second aspect, the invention relates to a method for generating optimized mask layout data for photomasks. In the method in accordance with the second aspect, original data which define an original layout for the simulation of a lithography method are again prescribed. Proceeding from the original data, new data are calculated automatically or in another way. The new data define a new layout which is more similar, with regard to the geometry, to a mask that is produced or can be produced using the original data than to the original layout.
By way of example, the new data can be calculated by simulation of the method steps during the production of the mask. As an alternative, however, the method in accordance with the first aspect or in accordance with one of the abovementioned developments and refinements can also be used in order to define the new data.
It is an object of the second aspect of the invention to specify a simple method for generating optimized mask layout data for photomasks. Furthermore, the intention is to specify an associated apparatus and an associated program.
The invention in accordance with the second aspect proceeds from the consideration that taking account of the influence of the production process of the mask is only a partial step on the way to defining final mask layout data which are then actually used for the mask production. This is because the changes that occur through the production process in turn require a change of the original layout in a corrected layout. These changes have hitherto been carried out manually, but can also be automated.
In the invention""s method in accordance with the second aspect, corrected data are automatically defined proceeding from the new data in such a way that a corrected mask that is produced or can be produced using the corrected data is more similar, with regard to the geometry, to the original layout than to a mask that is produced or can be produced using the original data. Consequently, the original layout is considered as the aim of the production process of the mask. The reference to the original layout allows the definition of simple criteria for the automatic correction.
Alternatively or cumulatively, in the method in accordance with the second aspect, the corrected data are designed in such a way that the mask that is produced or can be produced using the corrected data has better lithographic imaging properties than a mask that is produced or can be produced using the original data. The lithographic imaging properties are of fundamental importance for the structure widths that can be achieved during the wafer exposure. The chip production yield can be considerably increased by virtue of the improved imaging with a mask produced from the optimized mask layout data. Moreover, it is thus possible to produce chips having a greater electrical performance, e.g. with regard to a higher clock frequency or a lower current consumption.
One criterion for the automatic correction is, in one development, the ratio of the dark areas and of the bright areas of the original layout. In the corrected layout, this ratio is to be preserved or changed by a prescribed value. And this is because the area ratio is initially changed in the new layout on the basis of the effects that are taken into account.
In one development, the corrected layout data are calculated on the basis of correction rules which are based on deviations in the geometry of a layout from a mask that is produced according to this layout or a mask that is modeled proceeding from this layout with simulation of the production process. The fact that the deviations in the geometry are taken into account already means that it is possible to define so many correction rules that the correction can be completely or almost completely automated.
In one refinement, the method discussed above in connection with the shortening value is used for defining the new data. In accordance with a correction rule, depending on the shortening value, a lengthening value is then determined for a reference structure at a position of the original layout. Depending on the lengthening value, the structure located at the same position in the corrected layout is subsequently lengthened in the longitudinal direction in comparison with the structure located at the same position in the new layout. The aim here is to approximate to the structure prescribed by the original layout at the same position. This is done in an iteration method, for example. However, it is also possible to use approximation specifications. A correction method is explained below with reference to FIG. 8.
In a further refinement, use is made of the method discussed above in connection with the radius or curvature value. In accordance with a further correction rule, depending on the radius or curvature value, a lengthening value is then determined for a reference structure at a position of the original layout. Depending on the lengthening value, the structure located at the same position in the corrected layout is subsequently lengthened in the longitudinal direction and/or in the transverse direction in comparison with the structure located at the same position in the new layout. In this refinement, instances of shortening which are brought about by the rounding of the corners are compensated for again.
In a next refinement, in accordance with a further correction rule, depending on the constriction value, a widening value is determined for a reference structure at a position of the original layout. The abovementioned development of the method in accordance with the first aspect of the invention is used for determining the constriction value. Depending on the widening value, the structure located at the same position in the corrected layout is then widened at least in sections transversely with respect to the longitudinal direction in comparison with the structure located at the same position in the new layout. What is achieved by this measure, despite the xe2x80x9cpeanutsxe2x80x9d effect, is that the corrected mask contains a structure having a constant or intended width.
In addition to the abovementioned refinements, other correction rules are also used for correcting the consequences of other effects. In this case, simple geometric relationships are utilized each time.
In one development, the lengthening and/or the widening is implemented whilst maintaining the form of the structure in the new mask. As an alternative, however, simple structures can also be attached in the course of the lengthening or widening. By way of example, if the intention is to correct instances of rounding, then small squares are attached to the structure of the original mask to the left and right of a central axis, in order to obtain the corrected structure. The structure thus obtained a serif-shaped configuration, as known previously from OPC methods (Optical Proximity Correctionxe2x80x94correction of proximity-induced diffraction effects). Known OPC methods take account, in particular, of the exposure process of the wafer. By contrast, the method according to the invention essentially takes account of the effects which are brought about by the mask writer and the mask production process.
The invention additionally relates to an apparatus, in particular a data processing system, for generating mask layout data for lithography simulation or for generating optimized mask layout data for photomasks. However, use is also made of circuit arrangements or special hardware in a data processing system. The apparatus is constructed in such a way that the method steps according to one of the methods in accordance with the first aspect or in accordance with the second aspect or in accordance with the developments thereof are implemented during operation. Thus, the technical effects mentioned above also apply to the apparatus.
Furthermore, the invention relates to a program having a command sequence that can be executed by a data processing system. The method steps in accordance with the first aspect or in accordance with the second aspect or in accordance with a development of one of these aspects are implemented during the execution of the command sequence. The program is held for example in a RAM module (Random Access Memory) in a programmable memory module, on a floppy disc or on a compact disc, abbreviated to CD.